1. Field of the Invention
The present invention relates to a flaky α-alumina crystal and a preparation method thereof. More particularly, the present invention relates to a flaky α-alumina crystal comprising aluminum oxide and zinc oxide, which is prepared by hydrolyzing an aluminum precursor solution containing a water-soluble flux and a zinc precursor solution to obtain a gel mixture and then aging, drying, calcining and crystallizing it under a specific reaction condition, and a preparation method thereof. Since the flaky α-alumina crystal of the present invention has a thickness of less than 0.5 μm, an average particle diameter of at least 15 μm and an aspect ratio of at least 50, it is useful as a high-quality pearlescent pigment material and filler for ceramic materials.
2. Description of the Related Art
Ceramic is a collective term referring non-metal inorganic solid materials prepared by heat-treatment at a relatively high temperature. In general, the ceramic provides special properties such as superior resistance to fire, acid and alkali. Of the ceramic materials, fillers and pearlescent pigments are required to be flaky and have uniform size and thickness, with little interparticle coagulation and smooth surface.
Improved thermal conductivity and mechanical strength are prerequisites for a good filler used as a ceramic material. For this, it is preferable that the filler is thin, has no interparticle coagulation for uniform dispersion and distribution, and also has a uniform particle size.
The important factors of a material for a pearlescent pigment are particle size, shape, surface property, refractive index, and the like. Since large and small particles have different proportions of reflection and transmission of light on the particle surface, uniformness in particle size is essential for a vivid and uniform color. Also, the particle size greatly affects the coloration of the pearlescent pigment because it is closely related with the wavelength of light. That is, the smaller the particle, the larger the surface area, thereby increasing the coloration and enhancing reflectivity, and offering a more vivid color. However, in coating metals or metal oxides, it is usually not easy to provide uniform coatings on them and thus results in decrease in aspect ratio, which then reduces the effect of light interference thus deteriorating glossiness of the resulting pearlescent colors. Therefore, in order to realize the various pearlescent colors caused by interference of light, the particle size needs to be sufficiently uniform. Besides, the pearlescent pigment material should be a transparent particle having uniform thickness and smooth surface. These two factors are greatly associated with obtaining the pearlescent color via coloration and cover-up. If the particles are thick or the surface is not smooth, most of light is reflected or scattered on the surface, and thus it becomes difficult to obtain the pearlescent color. Meanwhile, if the particles are coagulated or have non-uniform thickness, it is difficult to obtain various pearlescent colors when coated with metals or metal oxides. It is recommended that the flaky α-alumina crystal used as pearlescent pigment be transparent and has a thickness of less than 0.5 μm, an average particle size of 15-25 μm and a very smooth surface with no coagulation.
At present, lead carbonate, bismuth oxychldoride (BiOCl), natural mica, synthetic mica, etc., are used as a pearlescent pigment material. Of these, mica is also used as a modifier for improving ductility and mechanical strength of ceramic materials and as an additive for improving thermal conductivity.
Recently, methods of preparing flaky alumina prepared by hydrothermal method or by adding titanium dioxide have been reported.
The hydrothermal method is problematic in that the particle size is small and the aspect ratio is low. The method of using titanium dioxide as an additive provides superior prosperities for a pearlescent pigment. However, it is difficult to obtain a flaky alumina crystal having superior particle size distribution and dispersibility because aging is not performed after hydrolysis while calcinations is not performed before crystallization.
Generally, flaky alumina is formed as follows. Pseudo-boehmite, which is formed from hydrolysis of an aluminum salt solution, undergoes a phase transition to γ-alumina (γ-Al2O3) at a temperature of about 400° C. or higher. If the melt salt solution is heated to 1,200° C., a hexagonal, flaky crystal of α-alumina is obtained as a result. The flaky α-alumina crystal needs to have a thin and uniform plane and an aspect ratio (diameter/thickness) of at least 50 and be transparent, so that it can provide pearlescent gloss of various colors when coated with metals or metal oxides. However, because the flaky crystal prepared by the conventional method is rather thick and small, it is not good for being used as a pearlescent pigment.